1. Architectural Design, Distributed Systems Architectures
Lectures 17 and 18

2. Distributed Systems Architectures
Architectural Design - Establishing the overall structure of a software system
Topics covered:
System structuring
Control models
Modular decomposition
Multiprocessor architectures
Client-server architectures
Distributed object architectures
Architectural Design
Distributed Systems Architectures

3. Software architecture
The design process for identifying the sub-systems making up a system and the framework for sub-system control and communication is architectural design
The output of this design process is a description of the software architecture

4. Architectural design An early stage of the system design process
Represents the link between specification and design processes
Often carried out in parallel with some specification activities
It involves identifying major system components and their communications

5. Architectural design process
System structuring
The system is decomposed into several principal sub-systems and communications between these sub-systems are identified
Control modelling
A model of the control relationships between the different parts of the system is established
Modular decomposition
The identified sub-systems are decomposed into modules

6. Sub-systems and modules
A sub-system is a system in its own right whose operation is independent of the services provided by other sub-systems.
A module is a system component that provides services to other components but would not normally be considered as a separate system

7. Architectural models
Different architectural models may be produced during the design process
Each model presents different perspectives on the architecture:
Static structural model
Dynamic process model
Interface model
Relationships model

8. Architectural models
Static structural model that shows the major system components
Dynamic process model that shows the process structure of the system
Interface model that defines sub-system interfaces
Relationships model such as a data-flow model

9. Concerned with decomposing the system into interacting sub-systems
System structuring
Concerned with decomposing the system into interacting sub-systems
The architectural design is normally expressed as a block diagram presenting an overview of the system structure
(More specific models showing how sub-systems share data, are distributed and interface with each other may also be developed)

10. Packing robot control system

11. The repository model
Sub-systems must exchange data. This may be done in two ways:
Shared data is held in a central database or repository and may be accessed by all sub-systems
Each sub-system maintains its own database and passes data explicitly to other sub-systems
When large amounts of data are to be shared, the repository model of sharing is most commonly used (WHY???)

12. Repository model characteristics
Advantages
Efficient way to share large amounts of data
Sub-systems need not be concerned with how data is produced
Centralised management e.g. backup, security, etc.
Sharing model is published as the repository schema
Disadvantages
Sub-systems must agree on a repository data model. Inevitably a compromise
Data evolution is difficult and expensive
No scope for specific management policies
Difficult to distribute efficiently

13. Client-server architecture
Distributed system model which shows how data and processing is distributed across a range of components
Set of stand-alone servers which provide specific services such as printing, data management, etc.
Set of clients which call on these services
Network which allows clients to access servers

14. Film and picture library

15. Client-server characteristics
Advantages
Distribution of data is straightforward
Makes effective use of networked systems. May require cheaper hardware
Easy to add new servers or upgrade existing servers
Disadvantages
No shared data model so sub-systems use different data organisation. data interchange may be inefficient
Redundant management in each server
No central register of names and services - it may be hard to find out what servers and services are available

16. Abstract machine model
- Used to model the interfacing of sub-systems
Organises the system into a set of layers (or abstract machines) each of which provide a set of services
Supports the incremental development of sub-systems in different layers. When a layer interface changes, only the adjacent layer is affected
However, often difficult to structure systems in this way

17. ISO/OSI network model
Application

18. Control models
Are concerned with the control flow between sub systems. Distinct from the system decomposition model
Centralised control
One sub-system has overall responsibility for control and starts and stops other sub-systems
Event-based control
Each sub-system can respond to externally generated events from other sub-systems or the system’s environment

19. Centralised control
A control sub-system takes responsibility for managing the execution of other sub-systems
Call-return model
Top-down subroutine model where control starts at the top of a subroutine hierarchy and moves downwards. Applicable to sequential systems
Manager model
Applicable to concurrent systems. One system component controls the stopping, starting and coordination of other system processes. Can be implemented in sequential systems as a case statement

20. Call-return model

21. Real-time system control

22. Event-driven systems
Driven by externally generated events where the timing of the event is out with the control of the sub-systems which process the event
Two principal event-driven models
Broadcast models. An event is broadcast to all sub-systems. Any sub-system which can handle the event may do so
Interrupt-driven models. Used in real-time systems where interrupts are detected by an interrupt handler and passed to some other component for processing

23. Broadcast model
Effective in integrating sub-systems on different computers in a network
Sub-systems register an interest in specific events. When these occur, control is transferred to the sub-system which can handle the event
Control policy is not embedded in the event and message handler. Sub-systems decide on events of interest to them
(!!!) However, sub-systems don’t know if or when an event will be handled

24. Selective broadcasting

25. Interrupt-driven systems
Used in real-time systems where fast response to an event is essential
There are known interrupt types with a handler defined for each type
Each type is associated with a memory location and a hardware switch causes transfer to its handler
(!!!) Allows fast response but complex to program and difficult to validate

26. Interrupt-driven control

27. Modular decomposition
Another structural level where sub-systems are decomposed into modules
Two modular decomposition models covered
An object model where the system is decomposed into interacting objects
A data-flow model where the system is decomposed into functional modules which transform inputs to outputs. Also known as the pipeline model
If possible, decisions about concurrency should be delayed until modules are implemented

28. Object models
Structure the system into a set of loosely coupled objects with well-defined interfaces
Object-oriented decomposition is concerned with identifying
object classes,
their attributes and
operations
When implemented, objects are created from these classes and some control model used to coordinate object operations

29. Invoice processing system

30. Data-flow models
Functional transformations process their inputs to produce outputs
May be referred to as a pipe and filter model (as in UNIX shell)
Variants of this approach are very common. When transformations are sequential, this is a batch sequential model which is extensively used in data processing systems
Not really suitable for interactive systems

31. Invoice processing system

32. Distributed Systems Architectures
Architectural design for software that executes on more than one processor

33. Distributed systems
Virtually all large computer-based systems are now distributed systems
Information processing is distributed over several computers rather than confined to a single machine
Distributed software engineering is now very important

34. System types
Personal systems that are not distributed and that are designed to run on a personal computer or workstation.
Embedded systems that run on a single processor or on an integrated group of processors.
Distributed systems where the system software runs on a loosely integrated group of cooperating processors linked by a network.

35. Distributed system characteristics
Resource sharing
Openness
Concurrency
Scalability
Fault tolerance
Transparency
Distributed system disadvantages :
Complexity
Security
Manageability
Unpredictability

36. Distributed systems archiectures
Client-server architectures
Distributed services which are called on by clients. Servers that provide services are treated differently from clients that use services
Distributed object architectures
No distinction between clients and servers. Any object on the system may provide and use services from other objects

37. Middleware
Software that manages and supports the different components of a distributed system. In essence, it sits in the middle of the system
Middleware is usually off-the-shelf rather than specially written software
Examples
Transaction processing monitors
Data converters
Communication controllers

38. 1. Multiprocessor architectures
Simplest distributed system model
System composed of multiple processes which may (but need not) execute on different processors
Architectural model of many large real-time systems
Distribution of process to processor may be pre-ordered or may be under the control of a dispatcher

39. A multiprocessor traffic control system

40. 2. Client-server architectures
The application is modelled as a set of services that are provided by servers and a set of clients that use these services
Clients know of servers but servers need not know of clients
Clients and servers are logical processes
The mapping of processors to processes is not necessarily 1 : 1

41. A client-server system

42. Computers in a C/S network

43. Layered application architecture
Presentation layer
Concerned with presenting the results of a computation to system users and with collecting user inputs
Application processing layer
Concerned with providing application specific functionality e.g., in a banking system, banking functions such as open account, close account, etc.
Data management layer
Concerned with managing the system databases

44. Application layers

45. Thin and fat clients Thin-client model Fat-client model
In a thin-client model, all of the application processing and data management is carried out on the server. The client is simply responsible for running the presentation software.
Fat-client model
In this model, the server is only responsible for data management. The software on the client implements the application logic and the interactions with the system user.

46. Thin and fat clients

47. Thin client model
Used when legacy systems are migrated to client server architectures.
The legacy system acts as a server in its own right with a graphical interface implemented on a client
A major disadvantage is that it places a heavy processing load on both the server and the network

48. Fat client model
More processing is delegated to the client as the application processing is locally executed
Most suitable for new C/S systems where the capabilities of the client system are known in advance
More complex than a thin client model especially for management. New versions of the application have to be installed on all clients

49. A client-server ATM system

50. Three-tier architectures
In a three-tier architecture, each of the application architecture layers may execute on a separate processor
Allows for better performance than a thin-client approach and is simpler to manage than a fat-client approach
A more scalable architecture - as demands increase, extra servers can be added

51. A 3-tier C/S architecture

52. An internet banking system

53. 3. Distributed object architectures
There is no distinction in a distributed object architectures between clients and servers
Each distributable entity is an object that
provides services to other objects and
receives services from other objects
Object communication is through a middleware system called an object request broker (software bus)
However, more complex to design than C/S systems

54. Distributed object architecture

55. Advantages of distributed object architecture
It allows the system designer to delay decisions on where and how services should be provided
It is a very open system architecture that allows new resources to be added to it as required
The system is flexible and scaleable
It is possible to reconfigure the system dynamically with objects migrating across the network as required

56. Uses of distributed object architecture
As a logical model that allows you to structure and organise the system. In this case, you think about how to provide application functionality solely in terms of services and combinations of services
As a flexible approach to the implementation of client-server systems. The logical model of the system is a client-server model but both clients and servers are realised as distributed objects communicating through a software bus